David Anderson QC has warned that the UK terror threat is higher than it was six years ago

David Anderson QC said a sense of being "over the worst" when he started in 2011 had been a "false dawn".

In an interview with the Press Association, the independent reviewer of terrorism legislation said there is now a "wider range" of dangers.

"What we see now is not more people being killed in this country – we've been both lucky and skilful in that regard over the last 10 years," he said.

We have probably the best intelligence agencies in Europe

David Anderson QC

"We are seeing far more experienced terrorist fighters from this country in Syria than we ever saw in Afghanistan or east Africa or other theatres of war.

"And we are seeing a realisation on the part of the terrorists that they don't need sophisticated explosives plots to take great numbers of lives.

"People using automatic weapons, heavy goods vehicles, even knives, machetes, and securing all the publicity they could possibly want from deploying relatively simple weapons such as that."

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There may still be hundreds of Britons being radicalised in Syria

The potential return of jihadists after fighting alongside Islamic State has emerged as a challenge for security agencies in recent years.

Around 850 UK-linked individuals of "national security concern" have travelled to join the Syrian conflict, with just under half thought to have come back.

Mr Anderson said: "We've seen a lot of people return already from Syria, about a quarter of them have been prosecuted, and only a few have engaged in terrorist activity of any kind in this country, so far as we know.

"But we have to remember that the people who have returned already were not necessarily the most committed fighters and that hundreds of Britons remain in that theatre.

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"When Isis is defeated and loses its territory, as I'm sure at some stage will happen, they will be looking for somewhere else to go – whether that's back to their home countries or elsewhere in the world."

Mr Anderson, who is set to leave his role next month, described strong intelligence as the key to avoiding terrorism.

"We have probably the best intelligence agencies in Europe," he said. "We also have excellent co-operation on the whole between intelligence agencies and police.

"It's more difficult in this country to obtain firearms than it is on the continent of Europe. But there's no inevitability about this. We've developed some excellent skills in the fight against terrorism but we need a bit of luck as well."

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When ISIS loses territory they may change tack and become insurgents in their home countries

In other remarks, Mr Anderson:

Said the threat in Northern Ireland has remained severe, while there has been a rise in extreme right-wing violence

Repeated a call for improved ships manifests so port police can better carry out advance checks on those arriving in Britain

Described temporary exclusion orders – which are yet to be used despite being introduced to control the return of Britons suspected of involvement in terrorist activity abroad nearly two years ago – as 'cumbersome'

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Mr Anderson also warned that the threat in Northern Ireland remained severe

Said large internet and technology firms are coming to a "slow and sometimes painful realisation" that they are "not just neutral platforms for other people's communications" but also have an "editing function"

Said that, while "very strong" privacy safeguards are needed, intrusions into personal privacy from some forms of bulk data collection are "rarely as great as some would like to paint them"

Raised doubts about the Government's new counter-extremism proposals, saying the central problem is that "nobody seems able to define extremism in a sufficiently certain way for the application of these hard legal measures", adding: "I suspect that may prove an impossible task"

The official threat level for international terrorism in the UK is at severe, meaning an attack is "highly likely".

Security services and counter-terrorism units have foiled at least 10 attacks in the past two years.